She tells that a big cat is apparently living in the apartment complex. 

A grey and white kitten with black stripes wanders away from her mother and siblings one day while enjoying a walk outside. Lost in her surroundings, the kitten struggles to find her family and instead is found by a young boy, Youhei, and his mother. They take the kitten home, but pets are not allowed in their housing complex, so they try to find a new home for the kitten. This proves to be difficult, and the family eventually decides to keep the kitten, naming her "Chi". 
(Source: Wikipedia)

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Chi is a mischievous newborn kitten who, while on a leisurely stroll with her family, finds herself lost. Separated from the warmth and protection of her mother, she feels distraught. Overcome with loneliness, she breaks into tears in a large urban park meadow, when she is suddenly rescued by a young boy named Yohei and his mother. The kitty is then quickly and quietly whisked away into the warm and inviting Yamada family apartment …where pets are strictly not permitted.
 
(Source: Kodansha USA)
 
Note: Includes 2 extra chapters.

Otonari-san